I have the tamron non VC and also a Kenko pro 300 DG 2x tc.
I haven't used the combo a lot as there is some IQ loss (the 1.4x tc has negligible IQ loss IMO, so I use that more), but here are some samples:
40D, 70-200 + 2x @ 400mm, f5.6, 1/500, ISO 1000, elbows resting on shelf in WWT hide...